1. Field of the Invention
The invention relates generally to a method and system for performing a handoff in a wireless communication system.
2. Description of the Related Art
Recently, the reorganization of frequency bands has been an issue in making effective use of a frequency band used in a wireless communication system and in conforming the frequency band to the global standard. For example, though the 800 MHz band of the Japanese standard (hereinafter, referred to as an old 800 MHz band) is used for current communication using CDMA2000 1x (Code Division Multiple Access 2000 1x) in Japan, the frequency band is scheduled to be reorganized to a new 800 MHz band as the global standard. The new 800 MHz band is different from the old 800 MHz band in the upstream and downstream allocations of the frequency band to be used. Therefore, a multiband compatible portable terminal which can establish a communication in three frequency bands, the current frequency band (old 800 MHz band), the new frequency band (new 800 MHz band), and the high frequency band (2 GHz band), has been developed.
In the CDMA2000 1x, one base station has one or more sectors divided according to pilot codes, and one channel for communication is assigned to each sector. The portable terminal communicates with the base station with one of the channels, respectively. Hereinafter, a description will be made using an example in which one pilot code corresponds to one base station (in other words, only one channel is assigned to one base station).
The above-described multiband compatible portable terminal uses a plurality of frequency bands, as does the wireless communication system. When the portable terminal moves, for example, the currently communicating base station has to be switched to another base station of a different frequency band.
A function of switching a base station (communication channel) for continued communication is called a handoff (or hand over). The handoff may include a soft handoff and a hard handoff.
The soft handoff refers to a handoff performed without switching frequency, which is a handoff method inherent in CDMA systems. After simultaneously communicating with both the currently communicating base station (original handoff channel) and a new base station (handoff destination channel) temporarily, a switching process of the base stations is performed. Specifically, a portable terminal receives a neighboring base station list transmitted by the currently communicating base station, measures the signal strength of a pilot signal of each base station in the list, and transmits the measurement results to the currently communicating base station. The base station determines a handoff destination base station according to the received measurement results. The soft handoff is led by the base station.
As described above, in the soft handoff, since the portable terminal always communicates with one or more base stations, communication is not interrupted at a time of handoff. However, when two base stations cannot provide a service to the portable terminal at the same frequency, the soft handoff is impossible. Therefore, the soft handoff cannot be used in the case of switching the base station to one using a different frequency band. In such a case, the hard handoff is performed.
The hard handoff in the CDMA2000 1x method of the IMT-2000 standard includes two methods: DAHHO (Data Assisted Hard Hand Off) and MAHHO (Mobile Assisted Hard Hand Off).
The DAHHO is a method of carrying out a handoff to a second base station directly specified by a first base station without a search by a portable terminal for another frequency before the handoff.
In other words, in the DAHHO, since the first base station specifies the second base station unilaterally without referring to the information of the portable terminal, there is no guarantee that the portable terminal can acquire the second base station specified by the first base station. Therefore, a possibility of handoff error is comparatively high.
On the other hand, in the MAHHO, the portable terminal receives a neighboring base station (channel) list with different frequency bands specified by the currently communicating base station, switches the frequency to the frequency of the specified list only at the moment specified by a search instruction from the first base station, and searches for a base station (channel) in the list. After the portable terminal measures the energy intensity of the pilot signals from all the base stations (channels) in the list, the portable terminal switches the frequency to the original frequency again and reports the measured energy intensities of all the base stations (channels) to the first base station. The first base station determines the second base station from the base stations in the list according to the reported energy intensities and transmits to the portable terminal an instruction for switching the communication to the second base station. The portable terminal carries out the handoff according to the instruction.
In the above described MAHHO, when the portable terminal measures the energy intensities of the pilot signals of all the base stations in the base station list received from the first base station, the portable terminal temporarily switches the frequency used for communication with the first base station to the frequency used for communication with the base stations in the list, and thus communication with the first base station is interrupted temporarily and a momentary break occurs. Accordingly, there is a disadvantage in that a silence occurs during a voice call.
In order to prevent the momentary break, the number of times and the time for switching the frequency may be limited. In order to reduce the number of times and the time for switching the frequency, the number of the base stations (channels) which measure the energy intensities of the pilot signals may be reduced.
In the conventional MAHHO, however, if the number of the measured base stations (channels) is reduced, there is a possibility that an effective base station (channel) cannot be found as a handoff destination and that the portable terminal fails the handoff.
Therefore, a system and a handoff method for preventing a momentary break occurring in the MAHHO are required.